1. Field of the Invention
The present invention relates generally to a logging system for measuring magnetic permeability characteristics of formations through which a wellbore has been drilled. More particularly, the present invention relates to a system for imaging wellbore walls using arrays of induction coils configured to detect magnetic permeability variations in the wellbore walls.
2. Description of the Related Art
Modem petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the wellbore itself. The collection of information relating to conditions downhole, which commonly is referred to as "logging," can be performed by several methods. Oil well logging has been known in the industry for many years as a technique for providing information to a driller regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or "sonde" is lowered into the wellbore after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the wellbore. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed cylinder for housing the sensors, which hangs at the end of a long cable or "wireline." The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the wellbore, as the sonde is pulled uphole.
The sensors used in a wireline sonde usually include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. Various sensors have been used to determine particular characteristics of the formation, including nuclear sensors, acoustic sensors, and electrical sensors.
While wireline logging is useful in assimilating information relating to formations downhole, it nonetheless has certain disadvantages. For example, before the wireline logging tool can be run in the wellbore, the drillstring and bottomhole assembly must first be removed, or tripped, from the wellbore, resulting in considerable cost and loss of drilling time for the driller (who typically is paying daily fees for the rental of drilling equipment). In addition, because wireline tools are unable to collect data during the actual drilling operation, drillers possibly must make decisions (such as the direction to drill, etc.) without sufficient information, or else incur the cost of tripping the drillstring to run a logging tool to gather more information relating to conditions downhole. In addition, because wireline logging occurs a relatively long period after the wellbore is drilled, the accuracy of the wireline measurement can be questionable. As one skilled in the art will understand, wellbore conditions tend to degrade as drilling fluids invade the formation in the vicinity of the wellbore. In addition, the wellbore shape may begin to degrade, reducing the accuracy of the measurements.
Because of these limitations associated with wireline logging, there recently has been an increasing emphasis on the collection of data during the drilling process itself. By collecting and processing data during the drilling process, without the necessity of tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections "real-time", as necessary, to optimize drilling performance. For example, the driller may change the weight-on-bit to cause the bottomhole assembly to tend to drill in a particular direction. Moreover, the measurement of formation parameters during drilling, and hopefully before invasion of the formation, increases the usefulness of the measured data. Further, making formation and wellbore measurements during drilling can save the additional rig time which otherwise would be required to run a wireline logging tool.
Techniques for measuring conditions downhole, and the movement and location of the drilling assembly contemporaneously with the drilling of the well, have come to be known as "measurement-while-drilling" techniques, or "MWD." Similar techniques, concentrating more on the measurement of formation parameters of the type associated with wireline tools, commonly have been referred to as "logging while drilling" techniques, or "LWD." While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that the term encompasses both the collection of formation parameters and the collection of information relating to the position of the drilling assembly while the bottomhole assembly is in the well. The measurement of formation properties during drilling of the well by LWD systems improves the timeliness of measurement data and, consequently, increases the efficiency of drilling operations. Typically, LWD measurements are used to provide information regarding the particular formation in which the wellbore is traversing.
For a formation to contain petroleum, and for the formation to permit the petroleum to flow through it, the rock comprising the formation must have certain well known physical characteristics. One characteristic is that the formation has a certain measurable resistivity (or conductivity), which can be determined by inducing an alternating electromagnetic field into the formation by a transmitter coil arrangement. The electromagnetic field induces alternating electric currents (often called eddy currents) in the formation in paths that are substantially coaxial with the transmitter. These currents in turn create a secondary electromagnetic field in the medium, inducing an alternating voltage at the receiver coil. If the current in the transmitter coil is kept constant, the eddy current intensity is proportional to the conductivity of the formation. Consequently, the conductivity of the formation determines the intensity of the secondary electromagnetic field, and thus, the amplitude of the voltage at the receiver coil. This technique is commonly referred to as "induction logging". Resistivity may also be directly measured using electrodes, as is described in U.S. Pat. No. 4,468,623 "Method and apparatus using pad carrying electrodes for electrically investigating a borehole" by Gianzero et al. which issued Aug. 28, 1984.
The scale with which characteristic measurements are performed has tended to become finer as time goes on, in order to provide more detailed characterization of the lithography traversed by the wellbore walls. At the finer scales, the performance of the resistivity characterization is increasingly impaired when the resistivity of the drilling mud is significantly higher than the resistivity of the formation, as is often the case with oil-based synthetic muds. The formation of a mud cake on the wellbore wall can further impair the performance of the resistivity method. Acoustic methods encounter difficulty with heavy muds, or when a significant amount of gas is in the drilling mud.
It is desirable to have a high-resolution characterization technique which is not impaired by the resistivity of the drilling mud or the presence of a mud cake on the wellbore wall. Such a technique would advantageously be usable for imaging wellbore walls under a wide variety operating conditions, and would preferably be safe and inexpensive to operate.